1. Field of the Invention
The present invention relates generally to a ranging method for a broadband wireless access communication system, and more particularly to a handoff system using an initial ranging process in a broadband wireless access communication system for use with an OFDM (Orthogonal Frequency Division Multiplexing) scheme, and a method for controlling the same.
2. Description of the Related Art
Recently, many developers have conducted intensive research into the 4G (4th Generation) communication system as one of the next generation communication systems to provide a plurality of users with a specific service having a variety of QoSs (Quality of Services) at a transfer rate of about 100 Mbps. Currently, the 3G (3rd Generation) communication system provides a transfer rate of about 384 kbps in an outdoor channel environment having a relatively poor channel environment, and provides a maximum transfer rate of about 2 Mbps in an indoor channel environment having a relatively good channel environment. A wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system have been designed to provide a transfer rate of 20˜50 Mbps. Therefore, there has been newly developed a new communication system based on the 4G communication system to provide the wireless LAN and MAN systems for guaranteeing a relatively high transfer rate with mobility and QoS, and many developers have conducted intensive research into a high-speed service to be provided from the 4G communication system.
The wireless MAN system is suitable for a high-speed communication service because it has a wide coverage area and supports a high-speed transfer rate, but it does not consider the mobility of a subscriber station (SS) at all. As a result, there is no consideration of a handoff operation (i.e., a cell selection operation) caused by the high-speed movement of the SS. The communication system currently considered in IEEE (Institute of Electrical and Electronics Engineers) 802.16a and IEEE 802.16e specifications acts as a specific communication system for performing a ranging operation between the SS and a base station (BS).
FIG. 1 is a block diagram illustrating a broadband wireless access communication system using an OFDM/OFDMA (Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access) scheme. More specifically, FIG. 1 illustrates the IEEE 802.16a/IEEE 802.16e communication system.
The wireless MAN system, which acts as a BWA (Broadband Wireless Access) communication system, has a much wider coverage are and a much higher transfer rate than the wireless LAN system. When adapting the OFDM scheme and the OFDMA scheme to a physical channel of the wireless MAN system to provide the wireless MAN system with a broadband transmission network, this application system is called an IEEE 802.16a communication system. The IEEE 802.16a communication system applies the OFDM/OFDMA scheme to the wireless MAN system, such that it transmits a physical channel signal using a plurality of sub-carriers, resulting in high-speed data transmission. The IEEE 802.16e communication system has been designed to consider the SS's mobility in the IEEE 802.16a communication system, and there is no detailed specification yet for the IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16a/IEEE 802.16e communication system has a single cell structure, and includes a BS 100 and a plurality of SSs 110, 120, and 130, which are managed by the BS 100. Signal transmission/reception among the BS 100 and the SSs 110, 120, and 130 can be established using the OFDM/OFDMA scheme. A downlink frame structure for use in the IEEE 802.16a/IEEE 802.16e communication system will hereinafter be described with reference to FIG. 2.
FIG. 2 is a conceptual diagram illustrating the downlink frame structure for use in the BWA communication system using the OFDM/OFDMA scheme. More specifically, FIG. 2 illustrates a downlink frame structure for use in the IEEE 802.16a/IEEE 802.16e communication system.
Referring to FIG. 2, the downlink frame includes a preamble field 200, a broadcast control field 210, and a plurality of TDM (Time Division Multiplexing) fields 220 and 230. A synchronous signal (i.e., a preamble sequence) for acquiring synchronization between the BS and the SSs is transmitted via the preamble field 200. The broadcast control field 210 includes a DL(Downlink)_MAP field 211 and a UL(UpLink)_MAP field 213. The DL_MAP field 211 is used to transmit the DL_MAP message, and a plurality of IEs (Information Elements) contained in the DL_MAP message are shown below in the Table 1.
TABLE 1SyntaxSizeNotesDL_MAP_Message_Format( ){Management Message Type=2 8 bitsPHY Synchronization FieldVariableSee Appropriate PHYspecificationDCD Count 8 bitsBase Station ID48 bitsNumber of DL_MAP Element n16 bitsBegin PHY Specific section {See Applicable PHYsectionfor (i=1; i<=n; i++)For each DL_MAPelement 1 to nDL_MAP Information Element( )VariableSee corresponding PHYspecificationIf!(byte boundary) { 4 bitsPadding to reach bytePadding Nibbleboundary}}}}
With reference to Table 1, the DL_MAP message includes a Management Message Type field for indicating a plurality of IEs (i.e., transmission message type information); a PHY (PHYsical) Synchronization field established in response to a modulation or demodulation scheme applied to a physical channel in order to perform synchronization acquisition, a DCD (Downlink Channel Descript) count field for indicating count information in response to a DCD message configuration variation containing a downlink burst profile; a Base Station ID field for indicating a Base Station Identifier; and a Number of DL_MAP Element n field for indicating the number of elements found after the Base Station ID. Particularly, the DL_MAP message (not shown in Table 1) includes information associated with ranging codes allocated to individual ranging processes to be described later.
The UL_MAP field 213 is used to transmit the UL_MAP message. A plurality of IEs contained in the UL_MAP message are shown in Table 2 below
TABLE 2SyntaxSizeNotesUL_MAP_Message_Format( ){Management Message Type=3 8 bitsUplink Channel ID 8 bitsUCD Count 8 bitsNumber of UL_MAP Element n16 bitsAllocation Start Time32 bitsBegin PHY Specific section {See Applicable PHYsectionfor (i=1; i<=n, i++)For each DL_MAPelement 1 to nUL_MAP InformationVariableSee corresponding PHYElement( )specification}}}
Referring to Table 2, the UL_MAP message includes a Management Message Type field for indicating a plurality of IEs (i.e., transmission message type information); an Uplink Channel ID field a used Uplink Channel ID; a UCD (Uplink Channel Descript) count field for indicating count information in response to a UCD message configuration variation containing an uplink burst profile; and a Number of UL_MAP Element n field for indicating the number of elements found after the UCD count field. In this case, the uplink channel ID can only be allocated to a Media Access Control (MAC) sub-layer.
The TDM fields 220 and 230 indicate fields corresponding to timeslots allocated using a TDM/TDMA (Time Division Multiple/Time Division Multiple Access) scheme. The BS transmits broadcast information to be broadcast to SSs managed by the BS over the DL_MAP field 211 using a predetermined center carrier. The SSs monitor all the frequency bands having been previously allocated to individual SSs upon receipt of a power-on signal, such that they detect a pilot channel signal having the highest signal strength, i.e., the highest pilot CINR (Carrier to Interference and Noise Ratio). It is determined that the SS belongs to a specific BS that has transmitted the pilot channel signal with the highest pilot CINR. The SSs check the DL_MAP field 211 and the UL_MAP field 213 of the downlink frame having been transmitted from the BS, such that they recognize their own uplink and downlink control information and specific information for indicating a real data transmission/reception position.
The aforementioned UCD message configuration is shown below in Table 3:
TABLE 3SyntaxSizeNotesUCD-Message_Format( ){Management Message Type=08 bitsUnlink channel ID8 bitsConfiguration Change Count8 bitsMini-slot size8 bitsRanging Backoff Start8 bitsRanging Backoff End8 bitsRequest Backoff Start8 bitsRequest Backoff End8 bitsTLV Encoded Information forVariablethe overall channelBegin PHY Specific Section {for (i=1; i<n; i+n)Uplink_Burst_DescriptorVariable}}}
Referring to Table 3, the UCD message includes a Management Message Type field for indicating a plurality of Es (ie., transmission message type information), an Uplink Channel ID field for indicating a used Uplink Channel Identifier; a Configuration Change Count field counted by the BS; a mini-slot size field for indicating the size of the mini-slot of the uplink physical channel; a Ranging Backoff Start field for indicating a backoff start point for an initial ranging process, i.e., an initial backoff window size for the initial ranging process; a Ranging Backoff End field for indicating a backoff end point for the initial ranging process, i.e., a final backoff window size; a Request Backoff Start field for indicating a backoff start point for establishing contention data and requests, i.e., an initial backoff window size; and a Request Backoff End field for indicating a backoff end point for establishing contention data and requests, i.e., a final backoff window size. In this case, the backoff value indicates a kind of standby time, which is a duration time between the start of SS's access failure and the start of SS's re-access time. If the SS fails to execute an initial ranging process, the BS must transmit the backoff values for indicating standby time information for which the SS must wait for the next ranging process to the SS. For example, provided that a specific number of 10 is determined by the “Ranging Backoff Start” and “Ranging Backoff End” fields shown in Table 3, the SS must pass over 210 access executable chances (i.e., 1024-times access executable chances) and then execute the next ranging process according to the Truncated Binary Exponential Backoff Algorithm.
FIG. 3 is a conceptual diagram illustrating an uplink frame structure for use in a BWA communication system using an OFDM/OFDMA scheme. More specifically, FIG. 3 illustrates an uplink frame structure for use in the IEEE 802.16a/IEEE 802.16e communication system.
Prior to describing the uplink frame structure illustrated in FIG. 3, three ranging processes for use in the IEEE 802.16a/IEEE 802.16e communication system, i.e., an initial ranging process, a maintenance ranging process (also called a period ranging process), and a bandwidth request ranging process, will hereinafter be described in detail.
The initial ranging process for establishing synchronization acquisition between the BS and the SS establishes a correct time offset between the SS and the BS, and is used to control a transmission power (also called a transmit power). More specifically, the SS is powered on, and receives the DL_MAP message, the UL_MAP message, and the UCD message to establish synchronization with the BS in such a way that it performs the initial ranging process to control the transmission power between the BS and the time offset. In this case, the IEEE 802.16a/IEEE 802.16e communication system uses the OFDM/OFDMA scheme, such that the ranging procedure requires a plurality of ranging sub-channels and a plurality of ranging codes. The BS allocates available ranging codes to the SS according to objectives of the ranging processes (i.e., the ranging process type information). This operation will be described in more detail herein below.
The ranging codes are created by segmenting a PN (Pseudorandom Noise) sequence having a length of 215−1 bits into predetermined units. Typically, one ranging channel includes two ranging sub-channels, each having a length of 53 bits. PN code segmentation is executed over the ranging channel having the length of 106 bits, resulting in the creation of a ranging code. A maximum of 48 ranging codes RC#1˜RC#48 can be assigned to the SS. More than two ranging codes for every SS are applied as a default value to the three ranging processes having different objectives, i.e., an initial ranging process, a period ranging process, and a bandwidth request ranging process. As a result, a ranging code is differently assigned to the SS according to each objective of the three ranging processes.
For example, N ranging codes are assigned to the SS for the initial ranging process as denoted by a prescribed term of “N RC (Ranging Codes) for Initial Ranging”, M ranging codes are assigned to the SS for the periodic ranging process as denoted by a prescribed term of “M RCs for maintenance ranging”, and L ranging codes are assigned to the SS for the bandwidth request ranging process as denoted by a prescribed term of “L RCs for BW-request ranging”. The assigned ranging codes are transmitted to the SSs using the DL_MAP message, and the SSs perform necessary ranging procedures using the ranging codes contained in the DL_MAP message.
A period ranging process is periodically executed such that an SS, which has controlled a time offset between the SS and the BS and a transmission power in the initial ranging process, can control a channel state associated with the BS. The SS performs the period ranging process using the ranging codes assigned for the period ranging process.
A bandwidth request ranging process is used to enable the SS, which has controlled a time offset between the SS and the BS and a transmission power in the initial ranging process, to request a bandwidth allocation from the BS in such a way that the SS can communicate with the BS.
Referring to FIG. 3, the uplink frame includes an initial maintenance opportunity field 300 using the initial and period ranging processes, a request contention opportunity field 310 using the bandwidth request ranging process, and an SS scheduled data field 320 including uplink data of a plurality of SSs. The initial maintenance opportunity field 300 includes a plurality of access burst fields, each having real initial and period ranging processes, and a collision field in which there is a collision between the access burst fields. The request contention opportunity field 310 includes a plurality of bandwidth request fields each having a real bandwidth request ranging process, and a collision field in which there is a collision between the bandwidth request ranging fields. The SS scheduled data fields 320 each include a plurality of SS scheduled data fields (i.e., SS 1 scheduled data field˜SS N scheduled data field). The SS transition gap is positioned between the SS scheduled data fields (i.e., SS 1 scheduled data field˜SS N scheduled data field).
As described above, the IEEE 802.16a communication system has considered a fixed state of a current SS (i.e., there is no consideration given to the mobility of the SS) and a single cell structure. However, the IEEE 802.16e communication system has been defined as a system for considering the SS's mobility in the IEEE 802.16a communication system, such that the IEEE 802.16e communication system must consider the SS's mobility in a multi-cell environment. In order to provide the SS's mobility in the multi-cell environment, individual operations modes of the SS and the BS must be converted into others. However, the IEEE 802.16e communication system has not proposed a new method for improving the SS's mobility in the multi-cell environment. Therefore, an SS handoff system considering a multi-cell structure to provide the IEEE 802.16e communication system with the SS's mobility must be developed.